Ultrahigh molecular weight polyethylene (“UHMWPE”) is commonly used as a bearing material in orthopaedic implants, such as artificial hip joints. For example, UHMWPE has been used as a liner for the acetabular cup of the hip joint, where it acts as a bearing that facilitates the articulating movement of the femoral head within the acetabular cup.
While UHMWPE can exhibit relatively low rates of wear in most bearing applications, the relatively high elastic modulus of UHMWPE limits its use in applications which require contact with soft tissue, such as cartilage. For example, in such applications, the relatively hard UHMWPE can damage the cartilage after repeated articulation of the implant, especially if the point at which the cartilage and UHMWPE confront each other is subjected to relatively high loads. Thus, attempts have been made to provide implants or implant parts comprising UHMWPE that has been treated to reduce its elastic modulus. For example, U.S. Pat. No. 5,721,334 discloses a process for preparing a shaped article from UHMWPE having an elastic modulus of about 500-800 MPa using specific temperature and pressure profiles during the molding of the article. While the process described in the '334 patent may produce an article exhibiting an elastic modulus that is less than the elastic modulus of a similar article prepared in a conventional manner, the process can only achieve moderate decreases in the elastic modulus of the UHMWPE, which decreases may not be sufficient to address the concerns over soft tissue damage resulting from the use of UHMWPE in certain bearing applications.
Others have attempted to address the problem of providing suitable bearing materials by utilizing other wear-resistant polymers, such as polyurethanes, which have an elastic modulus that is less than the elastic modulus of UHMWPE. While the use of these polymers may provide a bearing surface that causes less trauma to the soft tissue which it confronts, some in the art have voiced concerns over the patient's response to the wear debris generated by these alternate bearing materials. For example, some have voiced concerns over possible tissue necrosis and/or osteolysis in response to the wear debris generated by polyurethane bearings.
A need therefore exists for a polymer composition that exhibits a lower stiffness relative to known polymer compositions comprising UHMWPE, as well as a method for making the same. The invention provides such a polymer composition and method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.